How an airfoil works - what'd I say wrong?

This is an answer I gave on Y!A. I got a lot of thumbs down and I wanna know what I said that was incorrect...
The typical airfoil (cross section of the wing) is flat on it's undersides and curved on the upper, with a hump closer to the leading edge. The camber of the airfoil deflects the airflow upwards thus... show more This is an answer I gave on Y!A. I got a lot of thumbs down and I wanna know what I said that was incorrect...

The typical airfoil (cross section of the wing) is flat on it's undersides and curved on the upper, with a hump closer to the leading edge. The camber of the airfoil deflects the airflow upwards thus less airfriction on the upper surface. It's purpose is to create high pressure on the underside and low pressure on the upperside to achieve lift. Since there is a stronger force on the underside, this produces lift. Airfoils are also used on propeller blades. When the flow is seperated due to a high angle of attack the plane stalls.

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Best Answer: A 'typical' airfoil that you are talking about is flatter on the bottom than on the top, but very few airfoils are flat on the bottom. We know what you are talking about though and the theory that Bernoulli's principle is the cause of lift. That is a not totally false, but Bernoulli's principle is only a small part of what causes an airfoil to generate lift.

Presumably you are aware that airshow pilots routinely fly for extended periods of time upside down. Doesn’t that make you suspicious that there might be something wrong with the story about curved on top and flat on the bottom?

Bernoulli's principle indicates that the air has to speed up to have equal transit time to get to the trailing edge at the same time as the air on the bottom, right? But in reality, the air on top arrives at the trailing edge BEFORE the air on the bottom.

Any airfoil will cause a turning action of the airflow, which will cause a reaction or aerodynamic force on the wing. Angle of attack is the greater force generator than the (equal transit time) theory.

Both Newton's forces and Bernoulli's ARE involved in the type of airfoil that you describe, but it's the angle of attack doing the heavy lifting. Both forces cause the airflow to turn, and that turning of the airflow is what generates a force we call lift.

Symmetric airfoils work just fine. In fact, neither Newton nor Bernoulli ever tried to explain the aerodynamic lift of an object.

An airplane would be unflyable if the coefficient of lift were determined solely by the shape of the wing. Since the amount of camber doesn’t often change in flight, there would be no way to change the coefficient of lift. The airplane could only support its weight at one special airspeed, and would be unstable and uncontrollable. In reality, the pilot (and the trim system) continually regulate the amount of lift by regulating the all-important angle of attack.

Your description is incorrect for several reasons. First, the "typical airfoil" is not flat on the bottom, but whether it is flat or not isn't really important to understanding how an airfoil works. While some airfoils are indeed flat bottomed, most are assymetrical. Some are even completely symmetrical. I won't explain how a symmetrical airfoil works, but it's a good brain-teaser of a question.

Second, there is not less friction on the upper surface of the wing. The air flowing over the top of the wing is accelerated due to the shape of the airfoil and angle of attack (in other words, the air has to travel further in an equal amount of time as air flowing beneath the airfoil), creating lower air pressure above the wing than below it. This is explained by Bernoulli's Law of fluid dynamics (look it up). High pressure is not "created" on the underside, relatively lower pressure is created on the upper side, and the differential creats "lift". This low pressure area above the wing does just as much pulling (or sucking is perhaps more descriptive) as the "pushing" caused by the relatively higher pressure air flowing across the bottom.

A basic airfoil (cross section of the wing) is flat on it's undersides and curved on the upper, with a hump close to the leading edge. The airflow is forced to travel a longer distance up and over the upper surface. This creates a higher pressure on the underside and a lower pressure on the upperside, since there is a stronger force acting on the underside, this produces lift. Airfoils are also used on propeller blades. When the air flow is subject to an acute high angle of attack on the leading edge of the wing the plane stalls.